The ocular impact of Viagra

Sildenafil citrate, otherwise known as Viagra, is used as an orally effective treatment for erectile dysfunction (ED) and pulmonary arterial hypertension. ED is the persistent inability to achieve or maintain an erection sufficient for sexual performance.1

Pulmonary arterial hypertension is characterised by an increase in pulmonary arterial blood pressure above the norm (=25mmHg at rest).2 Sildenafil is a potent and selective inhibitor of phosphodiesterase type 5 (PDE-5), with a low IC50 of 3.4nM.3 IC50 defines the molar concentration of the antagonist that produces 50 per cent of the maximum inhibitory response.4 A low IC50 indicates a potent inhibitor. Following oral administration of sildenafil, maximum plasma concentrations are achieved within 30-120 minutes. The half-life of sildenafil is approximately four hours, indicating rapid onset of action.5 Newer PDE-5 inhibitors include vardenafil and tadalafil.6 Tadalafil has a longer duration of action (half-life of 17.5 hours) compared to Vardenafil which is rapidly metabolised (half-life of 4-5 hours). These PDE5-inhibitors are highly plasma protein bound and are primarily metabolised by cytochrome P450 3A4 and to a lesser extent by CYP2C9 microsomal isoenzymes in the liver. Co-administration with known cytochrome P450 inhibitors can therefore affect the biotransformation and metabolism of these PDE-5 inhibitors, resulting in toxic plasma concentrations.5